1. Field of the Invention
The present invention relates to a laser cutting machine equipped with a sensor which measures a distance between a cutting head and a workpiece.
2. Description of the Background Art
FIG. 16 illustrates a conventional configuration of a laser cutting machine, having a base 1 for a cutting table, a table bed 2 movable in a linear direction on the base 1 (this linear axis is defined here as an X axis), a motor 3 which moves the table bed 2 in the X axis direction, a table 4 which can be moved in an orthogonal direction to the X axis by a motor, not shown, on the table bed 2 (this linear axis is defined here as a Y axis), an elevation body 5 which moves up and down in a direction orthogonal to the X and Y axes, respectively (this elevating axis is defined here as a Z axis), a cover 6 designed to protect the elevation body 5, a cutting head 7 fitted to the front end of the elevation body 5, a frame body 8 which supports the Z axis part, a laser oscillator 9 which outputs a laser beam, a laser beam guide cylinder 10, a laser cutting machine control apparatus 11 which accommodates a numerical control (NC) apparatus consisting of an interface with external signals, a CPU, memory, etc., and a workpiece 12.
The laser beam is output from the laser oscillator 9, guided through the laser beam guide cylinder 10, gathered by a lens in the cutting head 7, and applied to the workpiece 12 loaded on the table 4. At this time, a cutting gas is injected simultaneously. The laser cutting machine control apparatus 11 drives the X, Y and Z axes to move the relative positions of the cutting head and the table 4 and controls the laser beam output, etc., to cut the workpiece into a desired shape.
Also, in laser cutting, the focal point of the laser beam gathered by the lens must be set to a proper position relative to the workpiece 12. For this purpose, the cutting head 7 is fitted with a sensor which measures a distance between the cutting head and the workpiece to provide the laser cutting machine with a function which judges the excess of this signal data over a predetermined range or generates a signal indicating the contact of the head with the workpiece as improper and gives an alarm. Further, the laser cutting machine often has a copy function which moves the position of the cutting head 7 or the lens up/down under the control of the sensor signal to automatically set the focal point relative to the workpiece to a predetermined position.
FIG. 17 illustrates a conventional arrangement for a cutting control including the alarm detection function and the copy function, a main control section 13, a cutting machine control section 14, a position control section 15 and an alarm control section 16, in the numerical control (NC). Additionally, FIG. 17 illustrates a copy control section 17, a sensor data processor 18, a sensor 19 which measures a distance L, X axis servo control 20, Y axis servo control 21, Z axis servo control 22, servo motors 23, 24, 25 with position detectors of the X, Y and Z axes respectively. An apparatus 26 consisting of a laser oscillator, a whole cutting table and an alarm display 27 are also shown in FIG. 17.
Control exercised in accordance with an NC program to cut the workpiece will be described with reference to FIG. 17. The main control section 13 analyzes the cutting NC program on a block-by-block basis and gives information corresponding to program commands to the subsequent control sections. When the program commands are those given to the laser oscillator and cutting table, e.g., the on/off of the laser beam, cutting gas and workpiece clamp, the command information is given to the cutting machine control section 14. The cutting machine control section sends signals to the laser oscillator and cutting table 26 according to the command information. The laser oscillator and cutting table 26 are caused to execute laser beam output on/off, cutting gas solenoid valve on/off, etc., under the control of the command signals. Reversely, the signals of limit switches, various sensors, etc., from the laser oscillator and cutting table 26 are transmitted to the main control section 13 via the cutting machine control section 14, whereby the control apparatus is given the information on the status of the laser oscillator and cutting table 26.
When the program command is a position command, the main control section 13 gives the position control section 15 information such as a move position and travel speed. According to the information given, the position control section 15 calculates a travel distance, distributes it to the X and Y axes, and outputs travel commands to the servo control sections 20, 21. At the same time, the position control section 15 controls an actual position, etc., according to the travel output relative to the commanded travel distance, the remaining travel distance, and the information from the servo control sections. The servo control sections 20, 21 drive the servo motors 23, 24 to relatively move the cutting head 7 with respect to the workpiece. In this manner, the cutting head moves according to the shape defined by the program to cut the workpiece. Conversely, the position control section 15 transmits the information, such as the position, the output travel and the remaining travel distance, to the main control section 13.
The control of the copy function will now be described. When the program command is a copy function on/off command, the main control section 13 gives command information to the copy control section 17. When given a copy function ON command, the copy control section 17 compares information on the distance L input from the sensor data processor 18 with a preset predetermined distance and outputs a travel signal to the Z axis servo control section 22 to eliminate a difference therebetween. The servo control section 22 drives the servo motor 25 to move the cutting head 7 vertically. Among various types of sensors, the distance sensor 19 shown in the drawing is of a capacitance type. The sensor 19 outputs a signal corresponding to the distance L between the nozzle and the workpiece 12, and the sensor signal is fed back to the copy control section 17 via the sensor data processor 18. In this manner, when the distance L changes due to the distortion or the like of the workpiece, the sensor data changes and the Z axis position changes according to the change of the sensor data to keep the distance between the cutting head and the workpiece preset and predetermined. Conversely, the copy control section 17 transmits the copy status information to the main control section 13.
The control of the alarm detection function will now be described. When the signal received from the sensor 19 has exceeded a predetermined range or indicates contact of the head with the workpiece, the sensor data processor 18 judges that the positional relationship between the cutting head 7 and the workpiece 12 is unsatisfactory and gives an alarm signal to the alarm control section 16. Receiving the alarm signal, the alarm control section 16 informs the main control section 13 of the alarm, demands that cutting should be stopped, and displays the alarm on the alarm display 27. When the alarm is input, the main control section 13 stops the execution of the cutting program, commands the cutting machine control section 14 to switch off the laser beam, etc., commands the position control section 15 to stop axis movement, and commands the copy control section 17 to switch off copying. FIG. 18 is an alarm generation processing flowchart. In step S1, it is judged whether or not the signal from the sensor has exceeded the predetermined range or indicates an alarm such as contact with the workpiece. When the signal is the alarm signal, the alarm is generated in step S2.
In addition to the above program commands, the main control section 13 processes a subprogram call command, a cutting condition switching command and other commands according to the contents of the NC program, gives information to the corresponding control sections as required, and also receives information, such as command execution completion, from the corresponding control sections.
FIG. 19 illustrates an example of a conventional cutting program. When a piercing condition selection command is provided in block 1, the laser output of the laser oscillator, the cutting gas pressure electro-pneumatic proportioning valve of the cutting table, and the like are set to the given piercing conditions through the processing of the main control section 13 and the cutting machine control section 14 of FIG. 17. When a copy function ON command is provided in block 2, the copy control section 17 performs processing to cause the cutting head to move until the distance L from the workpiece reaches a preset piercing distance and thereafter move to maintain the preset distance. When the preset distance is reached, the main control section 13 executes a piercing command in next block 3 to transmit the start of piercing to the cutting machine control section 14 and command it to switch the cutting gas and the laser beam on. This causes the laser oscillator to switch on the laser beam and the cutting table to open a cutting gas solenoid valve, whereby boring at the start of cutting (piercing) is initiated. When a sensor provided on the cutting table 26, which is referred to as the piercing sensor, detects the completion of boring (that a hole has passed through the workpiece) the sensor transmits this information to the cutting machine control section 14, which then informs the main control section 13 of the piercing completion. On completion of piercing, block 4 is executed. In block 4, the laser output, etc., are set to cutting conditions. The preset copy distance is also switched to a cutting distance. When a move position command is given in block 5, the position control section performs processing to move the cutting head in the X and Y directions. Hereafter, a cutting condition change, a position movement and other commands are given up to block 98 and the workpiece is cut into a required shape under the commands. When a cutting end command is given in block 99, the main control section 13 commands the cutting machine control section to switch the cutting gas and the laser beam off. This ends a sequence of cutting, subsequently the position is moved, and the next cutting begins again with piercing.
The conventional laser cutting machine arranged as described above results in an alarm and stops cutting when information indicating the alarm of the sensor signal is given by the sensor data processor 18. However, the alarm signal from the sensor may be output when it is not required to stop cutting. For instance, the alarm signal may be provided by the sensor due to plasma, scattered spatters, workpiece projections, or accumulated spatters produced during boring at the start of cutting. Also, the alarm signal may be given when the workpiece cut is inclined or blown away at a cutting end point to affect the sensor. In such cases where cutting need not be stopped, the alarm was given to stop cutting.
FIG. 20 illustrates examples wherein the alarm signal is output although cutting need not be stopped. FIG. 21(a) shows that plasma 28 has been generated during piercing. In this case, the sensor, which cannot measure the distance L accurately due to the influence of the plasma, may output the alarm signal to generate an alarm during piercing. FIG. 21(b) shows that projection-like thin deformations have been produced in the periphery of a hole bored during piercing. In this case, the sensor outputs the alarm signal which indicates approach to or contact with the workpiece because of the influence of projections 29 during piercing or at the time of movement after piercing to generate the alarm. FIG. 21(c) shows that spatters have accumulated in the periphery of the hole during piercing. In this case, the sensor gives the alarm signal which indicates approach to or contact with the workpiece due to the influence of accumulations 30 at the time of movement after piercing to generate the alarm. FIG. 21(d) shows that the workpiece cut off has inclined. In this case, the sensor outputs the alarm signal which indicates approach to or contact with the workpiece 31 at the end of cutting to generate the alarm.
To cope with these problems, there is a method as in Japanese Laid-Open Patent Publication No. HEI 3-490 wherein the signals from the sensor are all obstructed during piercing, etc., so that they are not given to the control sections, whereby copy control is not exercised. However, this method does not provide an alarm and does not allow the copy function to be used, either. But, some types of cutting may be difficult if the copy function cannot be used. These include cutting methods where the set distance of the copy function is changed in small increments during piercing to lower or raise the cutting head little by little in order to drive the focal point of the laser beam into, or away from, the workpiece. Also, a cutting method wherein the set distance of the copy function is changed immediately after piercing to switch the cutting head height between cutting and piercing may be difficult if the copy function cannot be used. Since these cutting methods cannot be employed if the copy function is not executed, there is a problem that a cutting fault will occur. When the copy function is being implemented, an unauthorized signal is output from the sensor, which is often instantaneous.
FIG. 20 illustrates a sensor signal where an alarm signal is output due to plasma or spatters during piercing. As shown in this drawing, an alarm signal of short duration is often generated randomly. Since the duration is too short in this case to respond to the sensor signal at the response speed of the ordinary copy function, there is no problem in cutting if the copy function is implemented. Also, when the copy function is not executed in the case where spatters are accumulated as shown in FIG. 21(c), the cutting head does not rise relative to the accumulation at all, whereby when the cutting head front end passes the accumulation portions, it interferes with the accumulation, incurring damage to the workpiece and/or the cutting head. Further, if the accumulation closes up the hole at the front end of the cutting head where the laser beam passes, a cutting fault will occur and the scattered spatters will stick to the inner lens directly through the hole in the cutting head, causing the lens to deteriorate. Also, when the copy function is not performed in the case where the inclined workpiece approaches or contacts the cutting head, as in FIG. 21(d), the cutting head will easily interfere with the workpiece.
It is, accordingly, an object of the present invention to overcome said disadvantages by providing a laser cutting machine which does not give an unnecessary alarm so that the copy function can be used and cutting can be continued without being stopped.